The present invention relates to repeaters which have main functions of waveform equalization to compensate distortion of a digital signal caused during its propagation in a transmission line, reproduction of the digital signal and extraction of a timing signal for use in the reproduction of the digital signal.
Generally, a signal transmission network system such as represented by the token ring LAN (Local Area Network) includes a signal transmission line and a plurality of nodes connected to the transmission line. Each node equalizes/reproduces a signal received from an upstream neighbouring node and sends it via the transmission line to a downstream neighboring node. When the signal is transmitted from one node to a downstream neighboring node, it propagates over a length or distance of transmission line between the two nodes, i.e., a repeat length of transmission path.
Conventionally, the repeater of each node is provided with an equalizer which has, however, fixed equalization characteristics. Meanwhile, when a signal is a pulse signal, such as the differential Manchester code signal, for example, problems of jitter (phase step) will arise whenever the signal takes a long period repetitive pattern in which a burst of "0" bits or a burst of "1" bits (all-zero bit pattern or all-one bit pattern) is followed by a burst of "1" bits or a burst of "0" bits (all-one bit pattern or all-zero bit pattern). Namely, a transmission line generally has a response depending on the frequency of a transmission signal such that loss increases as the frequency increases for the same transmission distance, with the loss increasing as the distance increases (See FIG. 5). In the differential Manchester code mentioned above, the frequency for "0" is twice that for "1", and therefore, a transmitted pulse waveform of a "0" burst deviates in phase (in the zero-cross point where the waveform rises) from a transmitted pulse waveform of a "1" burst. This deviation is called jitter (pattern jitter). In this specification, a pulse code signal having a first frequency for a binary value "0" or "1" and a second frequency different from the first frequency for a binary value "1" or "0" as in the differential Manchester code is referred to as "a two-frequency code signal", and, generally, "a multi-frequency code signal". Assuming that an amount of jitter is defined as a deviation (lead or lag) of the time of rise for zero-crossing of a waveform of an all-one pattern ("1" burst) as measured from the time of rise for zero-crossing of a waveform of an all-zero pattern and that an amount of jitter per one repeating (repeater) is represented by .phi. (seconds), an amount of jitter .phi..sub.N (seconds) contained in a signal having passed through N repeaters will be given by the following equation. ##EQU1## where N is the number of repeaters, .xi. is the damping factor of a Phase-Locked Loop (PLL) circuit which is provided in the repeater for extracting timing signals as discussed in "JITTER-ACCOMODATING IN TOKEN-PASSING RING LANS", IBM J. RES. DEVELOP. Vol. 29, No. 6, November, 1985, pp. 580-586. In order to prevent occurrence of a signal reception error (the state in which received data cannot be taken in with a clock due to jitter) in the signal transmission system generated due to an accumulated jitter .phi..sub.N as represented by equation (1), an elastic buffer (the buffer size is .+-.E bits) is provided in the repeater of one of the nodes which acts as a master station (an active monitor station) in the system to ensure reception of data having jitter within a predetermined range. In this case, assuming that the transmission rate is represented by Mbps (T=1/M), the following equation must be satisfied ##EQU2## The value for E may be 3, for example, so that with M=4 Mbps .phi..sub.N .gtoreq.3.times.250=750 ns.
With the prior art system, the jitter amount contained in a signal sent from the repeater of a node varies depending on the transfer function H(f) of an equalizer provided in the repeater, as shown in FIG. 1, which illustrates a relation between the jitter amount .phi. contained in the signal sent to the transmission line from a repeater and the transmission length, i.e., the repeat length of transmission path. In FIG. 1 type a represents a jitter amount of a signal sent from a repeater having an equalizer designed to cover relatively small transmission lengths, while type b represents that from a repeater having an equalizer designed to cover relatively large transmission lengths. The equalizers provided in the repeaters exhibiting type a and type b responses have transfer functions H(f) as represented by the following equation in which the characteristic is such as to oppose the above-mentioned line characteristic (approximately a U-shaped characteristics, not shown): ##EQU3## where f.sub.3i, f.sub.1i and f.sub.2i represent the rise initiation time, the rise termination time and fall initiation time in the U-shaped characteristic of the equalizer with a relation f.sub.3i &lt;f.sub.1i &lt;f.sub.2i.
The f.sub.1i -f.sub.3i of the type a deviate slightly toward a higher frequency than toward the f.sub.1i -f.sub.3i of the type b.
In FIG. 1, the type a repeater performs substantially complete equalization for a line length at the most preferable equalization point a.sub.1 (such that the amount of jitter after equalization is 0), but equalization cannot be performed properly for a different line length and hence jitter remains. Similarly, for the type b repeater, jitter cannot be eliminated sufficiently for a line length at a point other than the most preferable equalization point b.sub.1. A region where .phi.&lt;0 and the zero-cross point in an all-one pattern leads the zero-cross point in an all-zero pattern is called an over-equalization region, and a region where .phi.&gt;0 and the zero-cross point in an all-one pattern lags the zero-cross point in an all-zero pattern is called an insufficient equalization region. The point where .phi.=0 and the zero-cross points in both the patterns coincide is the matched-filtered point or the most preferable equalization point.
FIG. 1 will now be analyzed from a standpoint of waveform equalization. FIGS. 2A and 2B show waveform equalizing characteristics (eye patterns) of fixed equalizers. FIG. 2A shows the characteristic for a transmission line having a length shorter than the most preferable equalization point length and FIG. 2B for a transmission line having a length larger than the most preferable equalization point length. The period of the all "1" pattern is twice that of the all "0" pattern. In the same Figure, forward- and reverse-phase waveforms are written together. As shown in FIGS. 2A. and 2B, in the case of jitter compensation by fixed equalizers, the eye height is degraded due to intersymbol interference for line lengths at points other than the line length at the most preferable point and hence the transmission quality (error rate) is lowered. If the line is short or in the overequalized region, the line loss is small, so that the input signal amplitude is large and the equalized signal amplitude is large with eye height degradation due to intersymbol interference being small (FIG. 2A). On the other hand, if the line is long or in an insufficiently equalized region, the line loss is large (especially, the line loss of high frequency components is very large), so that the input signal amplitude is small and the eye height degradation due to intersymbol interference so increases that the equalizer cannot sufficiently compensate for high-frequency components and the waveform becomes blunt (FIG. 2B). Unless the eye pattern is opened sufficiently, the S/N ratio (the signal-to-noise ratio) is deteriorated. Therefore, in order to prevent eye height degradation for transmission lines having lengths larger than the line length at the most preferable equalization point, it is necessary to set the most preferable point such that the corresponding line length is large. However, in that case, the jitter for line lengths in a certain range smaller than the line length at the most preferable equalization point is increased as shown in the type b in FIG. 1.
It may be conceivable that several fixed equalizers different in the equalizing characteristic are prepared and an equalizer satisfying a repeat length of transmission path is selected for use. However, in an actual transmission system, a signal may or may not pass a repeater to vary the line length and therefore, such concept would not be practical.
Generally, the elastic buffer size E and the number N of times of repeating (the number of repeaters) are determined according to system specifications. Then, an allowable jitter amount .vertline..phi..sub.max .vertline. per repeating (repeater or node) is necessarily determined. Therefore, in order to realize the allowable jitter amount, an increase in the repeat length of transmission path (line length) is limited.
As described above, according to the prior art, the jitter compensation per repeater is determined solely by the transfer function of the equalizer provided in the repeater, the jitter compensation in a network system was not satisfactorily applicable to other network systems in which the repeat length of transmission paths are different among nodes connected to a transmission line.